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1.
The schists and gneisses of the Kanmantoo Group in the eastern Mt. Lofty Ranges show a well marked foliation and lineation. The foliation seen in the field is usually parallel to the bedding. The micas have a preferred orientation parallel to the lineation, resulting in girdles or partial girdles in the fabric diagrams. Quartz does not appear to have any preferred orientation. The lineations plunge to the S.S.E. or N.N.W., the mean plunge being about 20° to the S.S.E. This agrees with the plunge of the fold axes measured in the field and with the plunge of major structures deduced from field mapping. The area is thus one in which all the lineations are “b” lineations.  相似文献   

2.
Automated electron backscattered diffraction (EBSD) was applied using a scanning electron microscope to obtain lattice preferred orientation (LPO) data for olivine in garnet peridotites of the Central Alps. As a reference frame, the LPOs of enstatite were also investigated. In the garnet peridotite at Cima di Gagnone (CDG), a weak foliation carrying a distinct lineation is present. The lineation is characterized by elongated enstatite, olivine and poikiloblastic garnet. Olivine shows a very unusual LPO with [100] normal to foliation and [001] parallel to lineation. Achsenverteilungsanalyse (AVA) maps demonstrate that [001] of olivine grains corresponds quite well to their maximum length axes which are preferentially parallel to the lineation. Numerous planar hydrous defects within (001) planes of olivine are marked by palisades of ilmenite rods and show a preferred orientation normal to lineation. Calculated P-wave velocities for CDG are fastest (8.32 km sу) normal to foliation with a relatively low anisotropy (2.9%). Compared to mantle peridotites with the usual (010)[100] LPO where the fastest Vp direction is towards the lineation, the relationship between flow geometry and seismic anisotropy is significantly different at CDG. Several mechanisms for the formation of the LPO type at CDG are considered, with glide possible on (100)[001] of olivine. On the basis of field data as well as petrographic and petrologic evidence, it has been demonstrated that the CDG garnet peridotite formed by prograde metamorphism from a hydrous protolith at pressures and temperatures of about 3.0 GPa and 750 °C, respectively. The CDG LPO is interpreted to have formed during hydrous subduction zone metamorphism. The same interpretation may hold for the previously investigated olivine LPO at Alpe Arami, which is similar to that at the nearby CDG. The observed anomalous LPO is no proof for ultradeep (>3.0 GPa) conditions.  相似文献   

3.
A method of kinematic analysis of structures, microstructuresand mineral preferred orientations, initially devised in the study of peridotites, has been applied to crustal rocks bearing evidence of large strains produced in metamorphic environments. Three tectonic lineaments (Angers-Lanvaux, Montagne Noire and Maydan) were selected. They illustrate a general situation arising in continental crusts when they are deformed by ductile transcurrent fault systems.The Angers-Lanvaux structure is bilaterally symmetric; its dominant feature is the horizontal stretching lineation which is parallel to the fold axes. The foliation and slaty cleavage in the most surficial formations wrap around the axis of the whole structure. The folds in the slates away from the axis also exhibit axes parallel to the general trend, but no stretching lineations. These folds are attributed to crustal shortening in a direction normal to the ductile fault. In the Montagne Noire recumbent folds are thrusted away from the axis of the structure over at least 25 km. The metamorphism is also centered on the structure and symmetrically reduced away from it. The core of the structure is occupied by a strongly lineated orthogneiss, cut by a late intrusive granite. The Maydan axial zone displays clear evidence of partial melting at various scales within the deformed gneisses: (1) in gashes perpendicular to the stretching lineation which in these anatectic formations tends to plunge at more than 45°; (2) in bands of deformed pegmatites; and (3) possibly in granites which on the one hand intrude the surrounding formations and on the other converge with increasing deformation on the fault zone. The quartz preferred orientations and microstructures in quartzite layers from Angers indicate that the plastic flow plane and direction lie, respectively, close to the foliation and lineation, the slight departure is ascribed to a flow with a rotational shear component.All this suggests a general model for the origin of such ductile zones. The horizontal relative displacement of crustal blocks along a ductile band is responsible for its overall steeply dipping foliation and horizontal lineations. Viscous heating progressively tends to concentrate the plastic flow along its axis. It is also responsible for the development of metamorphism and of anatexis at depth; the partially melted rocks tend to rise, building at shallower depth the arched structure in the axis of the ductile zone, with a continuing flowage parallel to this axis probably now in the solid state; they can also intrude the surrounding terrain as undeformed batholiths. The folds parallel to the stretching lineation in the axial zone are explained by the fact that, due to the escape of anatectic melts, the formations at depth flow in a narrowing channel. The upwelling of the axial structure induces a compression with folding in the surrounding sedimentary formations and gravity nappe sliding away from the axis.  相似文献   

4.
Schistose high-grade hematite orebodies (>64 wt % Fe) in the Iron Quadrangle, Minas Gerais, were formed in shear zones by hydrothermal alteration of the Paleoproterozoic Cauê BIF during the Transamazonian orogenesis. The ore is comprised of platy hematite (specularite) grains that define the foliation and overprint a relict banded martite-hematite fabric resembling, at first sight, a mylonite. The EBSD analyses of a m-scale schistose orebody from the Pau Branco mine show that specularite grew as elongated plates with the (00.1) plane parallel to the foliation. The population of the measured grain aspect ratio (GAR) is homogenous in different scales, and the longest axes of the crystals align with the stretching lineation (L//X) building continuous domains, or anastomose around stretched iron oxide aggregates and rootless fold hinges. The pole figure of the (00.1) plane shows usually a maximum centered on the pole of the foliation Z often elongated on a girdle perpendicular to the lineation L. The {10.4} pole figure has the configuration of a symmetric cleft girdle and the corresponding {11.0} and {10.0} pole figures present well developed girdles parallel to the foliation with an elongated maximum centered on X. Microstructures associated with crystal-plastic behavior and dynamic recrystallization are missing and the fabric of the orebody probably results from precipitation of strain-controlled oriented hematite plates and anisotropic syntaxial growth of favorably oriented grains with the intervention of hydrothermal fluids during Fe enrichment. The shear zone provided pathways for the percolation of mineralizing fluids under temperatures that varied from 140 to 350 °C or higher, under ductile or ductile–brittle conditions. The orthorhombic fabric and CPO (crystallographic preferred orientation) of the ore nevertheless contrast with the asymmetry of simple shear as observed in the torsion experiments by Siemes et al., 2010, Siemes et al., 2011, probably due to volume loss and possibly a flattening component of deformation in the ore zone.  相似文献   

5.
Cataclastic lineations   总被引：1，自引：0，他引：1  
This paper describes a stretching lineation developed on the S-foliations of cataclasites and fault gouges from the Akaishi Tectonic Line (ATL), central Japan. The lineation is called a ‘cataclastic lineation’, and appears to result from homogeneous laminae flow of cryptocrystalline matrix clay minerals and the preferred orientation of fractured porphyroclasts. Slickenlines overprint these textures forming cataclastic lineations. These two kinds of linear structures are developed at large angles to one another.The orthogonal projection of the perpendicular of the intersection of S-foliations, C-surfaces and R1 shears (B-axis) onto the C-surface, which defines the shear direction, is parallel to the orthogonal projection of cataclastic lineation on the C-surface. In addition, planes cut parallel to the cataclastic lineation and perpendicular to the C-surface show distinctive asymmetric structures indicating left-lateral shear, which coincides with the main fault movement of the ATL as defined by displacement of tectonic units, S-C-surfaces and R1 shears. In contrast, planes parallel to the slickenlines and perpendicular to the C-surface show no asymmetric fabrics. Thus, cataclastic lineations are considered to indicate the shear direction of the ATL during formation of the cataclasites, whereas the slickenlines indicate shear directions of later, superficial movements along the ATL which have no relationship to the main shear direction.  相似文献   

6.
In the Ormiston Nappe Complex, west of Alice Springs, central Australia, a deformed zone up to 0.7 km thick is developed in the sedimentary Heavitree Quartzite. The deformed zone is adjacent to a major thrust fault and is defined by mylonitic foliation, which is parallel to the thrust plane and by isoclinal folds. Recognition of original detrital quartz grains allows strain ellipsoids to be measured across the zone. The strain generally plots in the flattening field and many specimens show pure flattening strain. The mylonitic foliation is an axial-plane structure to the folds and is parallel to the XY-plane of the strain ellipsoid. A quartz elongation lineation may be present within the foliation and is parallel to the principal extension direction (the X-axis) of the strain ellipsoid.Strain is accommodated principally by intracrystalline plastic deformation of the quartz grains. In detail the strain is not homogeneous and may vary even between adjacent grains of the same specimen. Quartz optic axis fabrics reflect this strain inhomogeneity. If the strain ellipsoid is an oblate spheroid, c-axes lie in small-circle girdles about the principal shortening axis (the Z-axis). With general triaxial strain, the c-axes lie in a great-circle girdle or girdles which intersect the foliation parallel to the intermediate strain axis (the Y-axis) and lie symmetrically about the Z-axis. A random population of grains from a specimen often shows a composite c-axis pattern between these two types.With approach to the thrust there is an increase in the amount of strain within the specimens. The increasing strain correlates with an increase in the degree of c-axis preferred orientation of the deformed detrital grains and in the amount of new strain-free grains present in the deformed quartzite. Adjacent to the thrust the quartzite is completely composed of polygonal new grains. The new grains probably formed under syntectonic conditions caused by movement along the thrust. The bulk of the new grains developed by increasing misorientation between the subareas of an initially polygonized old grain. There is no evidence of any marked host control on new-grain orientation, but new grain c-axis plots are generally similar to the corresponding old-grain plots from the same specimen.  相似文献   

7.
Abstract

The fabric and the anisotropy of magnetic susceptibility of the Cabo Ortegal eclogite (NW Spain) are studied. These mafic rocks were metamorphosed and deformed under high pressures and temperatures between 390 and 370 Ma in a subduction/collision tectonic setting. Massive eclogite slices and deformed eclogite in shear zones have bulk magnetic susceptibilities of 31 to 82·10?5 S.I. and 28 to 75·10?5 S.I., respectively. The paramagnetic mineral fraction is the principal magnetic susceptibility carrier. This fraction includes notably garnet and clinopyroxene as matrix minerals, and ilmenite and rutile as accessory constituents. Though magnetic anisotropy degree varies between 3.1 % and 6.6 %, variations of this parameter in each rock type are marked. In the deformed eclogite, magnetic lineation (Kmax) and the pole to the magnetic foliation (Kmin) are coaxial and coincident with macroscopic petrofabric elements (foliation and lineation). In the massive eclogite, the magnetic fabric is dispersed along the principal structural planes and inversions are associated with samples with small degrees of anisotropy. The anisotropy of magnetic susceptibility is interpreted as being due to the crystallographic preferred orientation and spatial organisation of the polymineralic aggregate. Relating the evolution of the symmetry of magnetic fabric to the symmetry of petrofabric or deformation is rather precluded since susceptibility has multiple origins and bulk magnetic fabric is due to minerals of different symmetry. © Elsevier, Paris  相似文献   

8.
Blueschist-facies rocks of the central Seward Peninsula cropout over 8000 km2. Protoliths were Lower Paleozoic-Precambrian(?) shallow-water miogeoclinal sediments that were metamorphosed during the Middle Jurassic. Thermobarometric estimates yield ‘peak’ metamorphic conditions of 10–12 kbar at 460 ± 30°C. Crystallization of blueschist-facies minerals was synkinematic with development of a transposition foliation. This foliation is parallel to lithologic contacts and is axial planar to recumbent mesoscopic isoclinal folds. These folds are refolded by larger scale recumbent tight to isoclinal folds. Both fold sets have hinges parallel to a well-developed N—S stretching lineation. Sheath folds are also present. The long axes of the sheath folds also parallel the stretching lineation. This deformation was non-coaxial as indicated by microstructures and quartz c-axis fabrics. Folds nucleated, then rotated into parallelism with the stretching direction. Kinematic indicators show unequivocal top-to-the-north shear sense, compatible with blueschist formation during mid-Jurassic collision between the Brooks Range continental margin and a N-facing island arc (Yukon-Koyukuk). Convergence of these two plates is believed to have been nearly N—S (in present co-ordinates).  相似文献   

9.
We have measured P- and S-wave velocities on two amphibolite and two gneiss samples from the Kola superdeep borehole as a function of pressure (up to 600 MPa) and temperature (up to 600 °C). The velocity measurements include compressional (Vp) and shear wave velocities (Vs1, Vs2) propagating in three orthogonal directions which were in general not parallel to inherent rock symmetry axes or planes. The measurements are accompanied by 3D-velocities calculations based on lattice preferred orientation (LPO) obtained by TOF (Time Of Flight) neutron diffraction analysis which allows the investigation of bulk volumes up to several cubic centimetres due to the high penetration depth of neutrons. The LPO-based numerical velocity calculations give important information on the different contribution of the various rock-forming minerals to bulk elastic anisotropy and on the relations of seismic anisotropy, shear wave splitting, and shear wave polarization to the structural reference frame (foliation and lineation). Comparison with measured velocities obtained for the three propagation directions that were not in accordance with the structural frame of the rocks (foliation and lineation) demonstrate that for shear waves propagating through anisotropic rocks the vibration directions are as important as the propagation directions. The study demonstrates that proper measurement of shear wave splitting by means of two orthogonal polarized sending and receiving shear wave transducers is only possible when their propagation and polarization directions are parallel and normal to foliation and lineation, respectively.  相似文献   

10.
New data suggest syn-convergent extrusion and polyphase tectonics followed by late Variscan extension in the Strudengau area of the southern Moldanubian zone in Austria. The tectonic history can be summarized as follows: (1) The oldest ductile event is observed in HT/LP metamorphic pelitic gneisses, which preserve E-dipping foliation planes (D1-fabric) with NW–SE-trending lineations. (2) The overlying gneisses record HT/HP conditions with decompression-induced anatexis in the central part of the domain. These gneisses exhibit N–S trending, horizontal lineations along steep-dipping foliation planes (D2-fabric) crosscutting the D1-fabric of the pelitic gneisses. Along the margin, these rocks have been strongly mylonitized under amphibolite facies conditions (D2). D2 is interpreted as a significant vertical shear zone, which juxtaposes the HT/LP rocks against the orogenic lower crust. (3) Lastly, the whole area is overprinted by localized shear zones (D3-fabric) with top-to-the-NW kinematics. This newly discovered Strudengau shearing event is associated with isoclinal folding that possesses axial planes parallel to the mylonitic foliation and fold axes parallel to the stretching lineations. Initial mylonitization occurred under greenschist facies, representing the latest ductile event of the Strudengau area. The new geochronological data presented here indicate a narrow time frame (c. 323–318 Ma) for the D3 deformation. Therefore, this event is contemporaneous with the intrusion of the granites of the South Bohemian Batholith (330–310 Ma). The nearby South Bohemian Batholith and generally steep dyke swarms in the Strudengau area and to the north trend in a NE–SW preferred orientation, interpreted to be D3-synkinematic magmatism. In a regional context, the NW–SE stretching during D3 together with the synkinematic intrusion of dykes is associated with late orogenic extension in the Austrian Moldanubian Zone. Kinematic data of brittle normal faults and tension gashes are consistent with NW–SE-oriented extension under cooler conditions.  相似文献   

11.
Permian volcanic sediments at Bluff have been strained and thermally metamorphosed by Permian intrusives to metasediments of hornblende—hornfels facies. Quartz, which crystallised as a secondary mineral during metamorphism, has an unusual preferred orientation with c-axes either forming paired maxima in the plane containing the lineation (=maximum principal strain axis = direction of extension) and the perpendicular to schistosity (=minimum principal strain axis = shortening direction) or a broad maximum parallel to the lineation; the paired maxima are approximately 30° either side of the lineation. Some quartz grains are markedly elongate parallel to the lineation, and according to hypotheses of preferred orientation involving crystal plasticity, there should be some correlation between the shape of such grains and their c-axis orientations. Grain-size and shape analysis of Bluff quartz demonstrate that no such correlation exists; the analyses show that the preferred orientation results from oriented nucleation in the residual stress field immediately following the bulk straining of the rocks, with the distribution of c-axes as predicted by Kamb's hypothesis (1959). The time relationships of rock deformation, thermal metamorphism, and nucleation and growth of quartz are discussed.  相似文献   

12.
The E-W running Salem-Attur shear zone demarcates the tectonic boundary between Archaean Dharwar Craton in the north and Proterozoic Southern granulite terrane in the south. This study reveals that the shear zone is a low angle thrust. The thrust zone is around 10 m thick and it merges with the main shear zone along the strike. The thrust is developed on charnockite near Odyarpatti, which is retrograded into schists. Further, it is marked by gently dipping mylonitic foliation and subhorizontal lineation. The S-C fabric, mantled porphyroclasts and intragranular faults indicate northeasterly slip along the thrust. Recumbent shear folds SF1 are developed within the thrust zone. The thrust has been folded by late stage F2 fold which has brought variation in the orientation of the mylonitic foliation from subhorizontal to vertical attitude; the mylonitic lineations have been rotated to subvertical orientation also. Additionally, the F2 crenulations and shear cleavages and intersection lineations are superimposed on the mylonitic fabric. Thrusting along the Salem-Attur shear zone is probably the cause for upliftment of the charnockites to the upper crust. Post-upliftment stage has witnessed brittle deformation in the form of development of shear fractures in NNE-SSW and E-W directions. Pseudotachylites are emplaced along these fractures.  相似文献   

13.
The origin of dome-and-keel structural geometries in Archean granite–greenstone terrains appears to lack any modern analogues and is still poorly understood. The formation of these geometries is investigated using structural and anisotropy of magnetic susceptibility (AMS) data for the Chinamora batholith in Zimbabwe. The roughly circular-shaped batholith is surrounded by ca. 2.72–2.64 Ga greenstones. The batholith granitoid suites have been divided on the basis of their ages and fabric relationships into four distinct units: (i) banded basement gneisses; (ii) granodioritic gneisses; (iii) equigranular granites; and (iv) central porphyritic granites. In the gneissic granites a partial girdle (N–S) of poles to the magnetic foliation is developed that has been folded around a consistent, flat lying magnetic lineation plunging at shallow angles to the E or W. In the equigranular granites, the magnetic lineation generally plunges to the NW. The magnetic foliation has a variable strike, no clear trends can be distinguished. The AMS measurements of the porphyritic granite revealed a NW–SE striking foliation and showed subhorizontal magnetic lineations. The magnetic foliation is subparallel to the macroscopic foliation. Wall rocks are moderately inclined and show radial or concentric lineations, triaxial strain ellipsoids and kinematics that demonstrate off-the-dome sliding and coeval pluton expansion. The results of the observations do not point to a single emplacement process. Neither the observed structural data nor the magnetic fabric support a model envisaging spherically ‘ballooning’. It is argued that pluton diapirism played a major part in the formation of the fabrics in the gneisses, whereas the fabrics in the porphyritic granites reflect emplacement as laccolith-like sheets.  相似文献   

14.
The magnetic fabric of Late Miocene sediments from the southern Pannonian basin was studied on oriented samples collected from 19 geographically distributed localities. All of them are characterized by near-horizontal magnetic foliation plane after tilt correction, indicating weak deformation. Well-developed lineations were observed for 16 localities, which are interpreted as due to compressional/transpressional deformation, except from three localities, where the fabric must have been formed in an extensional setting. Comparison between the orientation of the map-scale folds and faults and magnetic lineation directions shows that magnetic lineation is either related to NNE-SSW directed compression, leading to the formation of folds or it can be connected to NW–SE or NNE-SSW trending dextral faults.  相似文献   

15.
Shape, size and orientation measurements of quartz grains sampled along two transects that cross zones of increasing metamorphic grade in the Otago Schist, New Zealand, reveal the role of quartz in the progressive development of metamorphic foliation. Sedimentary compaction and diagenesis contributed little to the formation of a shape‐preferred orientation (SPO) within the analysed samples. Metamorphic foliation was initiated at sub‐greenschist facies conditions as part of a composite S1‐bedding structure parallel to the axial planes of tight to isoclinal F1 folds. An important component of this foliation is a pronounced quartz SPO that formed dominantly by the effect of dissolution–precipitation creep on detrital grains in association with F1 strain. With increasing grade, the following trends are evident from the SPO data: (i) a progressive increase in the aspect ratio of grains in sections parallel to lineation, and the development of blade‐shaped grains; (ii) the early development of a strong shape preferred orientation so that blade lengths define the linear aspect of the foliation (lineation) and the intermediate axes of the blades define a partial girdle about the lineation; (iii) a slight thinning and reduction in volume of grains in the one transect; and (iv) an actual increase in thickness and volume in the survivor grains of the second transect. The highest‐grade samples, within the chlorite zone of the greenschist facies, record segregation into quartz‐ and mica‐rich layers. This segregation resulted largely from F2 crenulation and marks a key change in the distribution, deformation and SPO of the quartz grains. The contribution of quartz SPO to defining the foliation lessens as the previously discrete and aligned detrital quartz grains are replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and reduced alignment. Pressure solution now affects the margins of quartz‐rich layers rather than individual grains. In higher‐grade samples, therefore, the rock structure is characterized increasingly by segregation layering parallel to a foliation defined predominantly by mica SPO.  相似文献   

16.
将磁性组构方法用于甘肃白银厂矿区的构造分析.结果表明该区岩石的磁性组构与岩石组构具有同一分布规律.岩石的磁线理与岩石的拉长线理L~a近于一致,磁面理与第一期劈理S_1基本平行,并垂直于岩石的磁化率椭球的最小主轴,也垂直于应变椭球的最短主轴方位,故推断第一期劈理S_1的成因主要属压扁成因.同时受到后期的多次变形与褶皱迭加影响.S_1轴面的优选面状分布发生变位不大,表明该区多次构造变形主要来自早期的同一主应力场作用所致.矿区岩石的磁面理与应变椭球拉长轴面或第一期劈理面S_1存在某些的角度差,表明该区岩石的劈理面由于受后期多次构造变形影响,可能发生剪切运动或旋转机制.  相似文献   

17.
Geometrical relations between quartz C-axis fabrics, textures, microstructures and macroscopic structural elements (foliation, lineation, folds…) in mylonitic shear zones suggest that the C-axis fabric mostly reflects the late-stage deformation history. Three examples of mylonitic thrust zones are presented: the Eastern Alps, where the direction of shearing inferred from the quartz fabric results from a late deformation oblique to the overall thrusting; the Caledonides nappes and the Himalayan Main Central Thrust zone, where, through a similar reasoning, the fabrics would also reflect late strain increments though the direction of shearing deduced from quartz fabric remains parallel to the overall thrusting direction. Hence, the sense of shear and the shear strain component deduced from the orientation of C-axis girdles relative to the finite strain ellipsoid axes are not simply related nor representative of the entire deformation history.  相似文献   

18.
The 3D shape, size and orientation data for white mica grains sampled along two transects of increasing metamorphic grade in the Otago Schist, New Zealand, reveal that metamorphic foliation, as defined by mica shape‐preferred orientation (SPO), developed rapidly at sub‐greenschist facies conditions early in the deformation history. The onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic mica in favour of numerous new smaller grains of contrasting composition, higher aspect ratios and a strong preferred orientation. The metamorphic mica is blade shaped with long axes defining the linear aspect of the foliation and intermediate axes a partial girdle about the lineation. Once initiated, foliation progressively intensified by an increase in the aspect ratio, size and alignment of grains, although highest grade samples within the chlorite zone record a decrease in aspect ratio and reduction in SPO strength despite continued increase in grain size. These trends are interpreted in terms of progressive competitive anisotropic growth of blade‐shaped grains so that the fastest growth directions and blade lengths tend to parallel the extension direction during deformation. The competitive nature of mica growth is indicated by the progressive increase in size and resultant decrease in number of metamorphic mica with increasing grade, from c. 1000 relatively small mica grains per square millimetre of thin section at lower grades, to c. 100 relatively large grains per square millimetre in higher grade samples. Reversal of SPO intensity and grain aspect ratio trends in higher grade samples may reflect a reduction in the strain rate or reduction in the deviatoric component of the stress field.  相似文献   

19.
The Eastern Layered Series of the Rum Layered Suite, NW Scotland, comprises a sequence of sixteen (30–150 m thick) cyclic units. The upper troctolite–olivine gabbro parts of each of these units exhibit small-scale modal layering and a pervasive, layer-parallel mineral lamination that is often associated with ‘soft-sediment’ deformation structures. A sporadic, macroscopic magmatic lineation measurable on mineral lamination surfaces is also observed in places. Anisotropy of magnetic susceptibility (AMS) fabrics were studied in three of these cyclic units, (8, 9, and 10) in the northern part of the Eastern Layered Series. Magnetic fabrics measured in the troctolites and gabbros yield one dominant trend in which magnetic foliations parallel magmatic layering and magnetic lineations trend NW–SE and plunge gently. Magnetic fabrics measured for two detailed traverses through Unit 10 on the northern side of Hallival also yield one dominant trend, similar to that measured elsewhere in the Eastern Layered Series. However, toward the centre of Unit 10 in each traverse, magnetic lineations sometimes plunge approximately downdip (SW) on the magnetic foliation planes. The implications of these results are discussed with reference to previous textural and fabric observations on Rum. A model is suggested in which weak linear arrangements of cumulus olivine and plagioclase crystals are developed due to slumping and soft-sediment deformation of unconsolidated crystal mushes during central sagging of the Rum Layered Suite.  相似文献   

20.
The Strzelin Massif in SW Poland (Central European Variscides) records a protracted igneous evolution, with three main magmatic stages: (1) tonalitic I, (2) granodioritic and (3) tonalitic II/granitic. In the northern part of this Massif, the Strzelin intrusion proper comprises three successively emplaced rock types: a medium-grained biotite granite (303 ± 2 Ma), a fine-grained biotite granite (283 ± 8 Ma) and a fine-grained biotite-muscovite granite; based on field evidence, the third variety postdates both types of the biotite granites. The structural data from the three granites, including their parallel, approximately E–W striking and steeply dipping lithological contacts and ENE–WSW trending subhorizontal magmatic lineations, suggest that the emplacement of all three successive granite varieties was controlled by an active, long-lived strike-slip fault, striking ESE–WNW, with a dextral sense of movement. After the emplacement of the youngest biotite-muscovite granite, the intrusion underwent brittle extension which produced “Q joints” striking NNW–SSE to N–S and dipping at 55–70° WSW to W, and showing evidence of broadly N–S directed sinistral displacements. The structural observations, supported by new geochronological data, indicate that the internal structure of the composite granitoid intrusion, including the faint magmatic foliation and lineation, formed in a long-lived strike-slip setting, different from the subsequent, post-emplacement extensional tectonics that controlled the development of brittle structures.  相似文献   

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